Added some light explanation of config options to readme.

README.rst

@@ -55,6 +55,18 @@ Optionally, you can provide additional options to pass to SQLAlchemy's pool crea
         'recycle': 300
     }
 
+Here's a basic explanation of two of these options:
+
+* **pool_size** – The *minimum* number of connections to maintain in the pool.
+* **max_overflow** – The maximum *overflow* size of the pool. This is not the maximum size of the pool.
+
+The total number of "sleeping" connections the pool will allow is ``pool_size``.
+The total simultaneous connections the pool will allow is ``pool_size + max_overflow``.
+
+As an example, databases in the `Heroku Postgres <https://postgres.heroku.com>`_ starter tier have a maximum connection limit of 20. In that case your ``pool_size`` and ``max_overflow``, when combined, should not exceed 20.
+
+Check out the official `SQLAlchemy Connection Pooling <http://docs.sqlalchemy.org/en/latest/core/pooling.html#sqlalchemy.pool.QueuePool.__init__>`_ docs to learn more about the optoins that can be defined in ``DATABASE_POOL_ARGS``.
+
 Django 1.3 Support
 ------------------